How I can do a sequence diagram with GMF?
I have defined a metamodel in EMF, which has two elements: roles and interactions. The roles can be connected to other roles through interaction. As shown in the figure below:

I don't know how relevant it is for your use case, but note that Sirius (which is based on GMF internally) has support for the definition of sequence diagrams [1]. There are some constraints on the metamodel to make it work, but it has been used successfuly in Capella [2] and UML Designer [3] for example. Papyrus [4] also has sequence diagrams implemented on top of GMF.